Numerous methods have been used to detect information about objects, in particular about small objects placed on a surface. The following is by no means a complete review of the field, but rather describes a number of references that illustrate the state of the art.
U.S. Pat. No. 6,690,156, the disclosure of which is incorporated herein by reference, describes a system for identifying and determining the position of game pieces on a game board. One of the methods described uses an excitation coil surrounding the sensor to excite circuitry in the game pieces at its resonant frequency and a sensor on top of the board to acquire signals that are generated by the game pieces in response to the excitation. Based on the excitation and response, the position and identification of the game piece is determined.
US patent publication 2004/0095333, the disclosure of which is incorporated herein by reference, describes a tablet with a structure of orthogonal conductors immediately beneath the tablet surface but on top of a display screen. The tablet also includes a coil surrounding the surface. In operation, a stylus comprises a resonant circuit which is excited by the surrounding coil. A signal generated in response, by the stylus, is acquired by the conductors and is detected. Based on the presence of signals in some of the conductors, the presence and position of the stylus is determined. It is noted that while this reference deals mainly with a stylus and a tablet, the use of the system to determine information regarding game pieces (paragraph 116) is also suggested.
US patent publication 2004/0155871, the disclosure of which is incorporated herein by reference, describes a variant on the system of the previously discussed reference. In general, a dual purpose tablet is described utilizing the same general tablet structure as the previous reference. However, in a series of embodiments, the structure can be modified to also determine the presence of a finger on the surface using the same sensor used for the stylus' detection. In the second of these embodiments (described in paragraphs 163-174 and FIGS. 2 and 3) a signal is fed into conductors that lie in one direction. The finger introduces capacitive coupling between the orthogonal lines so that a signal is detected in those orthogonal lines that are adjacent or beneath the finger. This detected signal is used to determine the presence and position of the finger.
It is noted that each of these references describes using transparent conductors, such that the sensor can overlay an imaging screen such as an LCD screen.
Each of these references includes an extensive review of the prior art in the background section, which is not repeated here. However, this review is incorporated by reference together with the rest of these documents.